1. Field of the Invention
The present invention relates to a liquid discharge head for discharging liquid such as ink from a discharge port as a flying liquid droplet to form a record or an image on a recording medium, and a method for producing the same.
2. Related Background Art
The conventional ordinary liquid discharge head is provided with plural fine discharge ports for discharging liquid such as ink, liquid flow paths communicating with the discharge ports and a discharge energy generating element provided in each liquid flow path, and is so constructed as to provide the discharge energy generating element with a drive signal corresponding to recording information or image information thereby supplying the liquid in the liquid flow path corresponding to the discharge energy generating element with discharge energy to discharge the liquid as a flying liquid droplet from the discharge port thus achieving print recording or image formation. In such liquid discharge head of so-called side shooter type in which the liquid droplet is discharged in a direction perpendicular to the plane bearing the discharge energy generating element, there is adopted a configuration, as shown in FIG. 16, of forming a penetrating liquid discharge aperture 113 in a substrate 111 bearing the discharge energy generating element 112 and executing liquid supply from the rear side of the substrate. In FIG. 16, there are shown a liquid flow path 114, a discharge port 115 formed corresponding to each discharge energy generating element 112, a width d of the liquid supply aperture 113 at the top side of the substrate, and a distance L from the end of the liquid discharge aperture 113 to the center of the discharge energy generating element 112, wherein the liquid flow is represented by a chain line.
In the liquid discharge head of such side shooter type, a method of forming the liquid discharge aperture 113 by anisotropic etching of the Si substrate 111 is disclosed for example in the Japanese Patent Application Laid-open No. 9-11479. In such process for forming the liquid supply aperture, a Si substrate 101 having 100 orientation on the surface is employed as the substrate, and, as shown in FIG. 14A, etching mask layers 102 are formed on both sides of the Si substrate 101, and the etching mask layers 102 on the rear side is eliminated in a desired position for forming the penetrating hole constituting the liquid supply aperture (FIG. 14B). Then Si is anisotropically etching with anisotropic Si etching solution such as TMAH (tetramethylammonium hydroxide) aqueous solution whereby 111 crystalline surface of Si is exposed to form a penetrating hole 113 having a plane inclined by 54.7° to the substrate surface.
Si substrate is associated with unevenness in the size and density of defects therein, because of fluctuation in the Oi (interlattice oxygen) concentration among wafers and within wafer, present even in the stage of single crystal formation, and fluctuation in the thermal process among wafers and within wafer, applied in the course of semiconductor device formation.
In the presence of such unevenness in the size and density of defects in the Si substrate, the penetrating hole 103 formed by anisotropic etching becomes inversely tapered in the vicinity of the rear surface of the Si substrate 107, as shown in FIG. 14C. This is because the etching is not dependent on the crystalline orientation in an area having a relatively high density of the crystal defects in the vicinity of the rear surface (range of 20 to 150 μm from the rear surface) of the Si substrate 101. Also similar anisotropic Si etching is executed from the rear surface with etching masks of a same size over the wafer surface, the aperture width d of the penetrating hole 103 on the top surface fluctuates such as d1, d2, d3 as shown in FIG. 14C (d1>d2>d3 in the illustration) whereby the finished dimension of the penetrating hole varies depending on the location. Such dimensional fluctuation results from the unevenness in the etching rate based on the unevenness in the size and density of the defects, and the dimensional fluctuation of the penetrating hole constituting the liquid supply aperture amounts to 40 to 60 μm between the maximum and minimum values of the aperture width d within the same plane. The aperture width d of the penetrating hole is also influenced by the fluctuation in the thickness of the silicon substrate and in the concentration of the etching solution.
In the liquid discharge head of side shooter type prepared by forming the liquid supply aperture by anisotropic etching in the Si substrate, as shown in FIG. 15, there will result a fluctuation in the aperture width d of the liquid supply aperture 113 on the top side of the substrate bearing the discharge energy generating elements, even within a liquid discharge head of a single chip. Such fluctuation leads to a fluctuation in the distance L (cf. FIG. 16) from the end of the liquid supply aperture 113 to the discharge energy generating element 112. In FIG. 15, a solid line indicates the state of opening of the liquid supply aperture 113 on the top side in case of actual anisotropic Si etching from the rear side, while a chain line indicates the ideal opening state of the liquid supply aperture 113 calculated from the dimension of the etching mask. Also a broken line 117 indicates the aperture of the etching mask formed on the rear side of the substrate 111.
In the liquid discharge head provided with the liquid supply aperture involving such fluctuation in the aperture width on the top side, there will result variation in the distance L between the end of the liquid supply aperture and the discharge energy generating element and in the flow resistance for the liquid flowing in such portion, thereby resulting in a significant influence on the working frequency characteristics of the liquid discharge head.
As explained in the foregoing, in the method of forming the liquid supply aperture in which the aperture width thereof is determined by the etching mask on the rear side of the wafer, there results fluctuation in the aperture width d of the liquid supply aperture and in the distance L between the end of the liquid supply aperture and the discharge energy generating element because of the fluctuation in the thickness of the silicon substrate and in the concentration of the etching solution and also because of the unevenness in the size and density of the defects in the silicon substrate, thereby rendering the liquid supply characteristics of the discharge energy generating elements uneven and causing significant influence on the operating frequency characteristics of the liquid discharge head.
Consequently there is desired technology for forming the liquid supply aperture, capable of improving the precision of the distance between the end of the liquid supply aperture and the discharge energy generating element.
In this regard, the U.S. Pat. No. 6,143,190 discloses a method of forming a through hole in a silicon substrate comprising (a) a step of forming, in a portion on the surface of the substrate where the through hole is to be formed, a sacrifice layer enabling selective etching with respect to the material of the substrate, (b) a step of forming a passivation layer having etching resistance on the substrate so as to cover the above-mentioned sacrifice layer, (c) a step of forming an etching mask layer having an aperture corresponding to the sacrifice layer on the rear surface of the substrate, (d) a step of etching the substrate by crystal axis anisotropic etching from the above-mentioned aperture until the sacrifice layer becomes exposed, (e) a step of eliminating the sacrifice layer by etching from the portion exposed by the aforementioned substrate etching step, and (f) a step of eliminating a part of the passivation layer thereby forming a through hole.
In the above-mentioned patent application, the sacrifice layer is formed either by forming and patterning a polysilicon layer or by epitaxial growth of silicon, but the formation and pattern of polysilicon layer require an additional mask for pattern and may result in aberration in patterning. Also epitaxial growth of silicon requires a complex apparatus and cannot be easily achieved with a low cost.